The present invention relates to an automatic tensioner for tensing a belt such as a timing belt for an automotive engine and more particularly to a spring loaded screw type tensioner.
Japanese Patent Publication No. 58-24666 discloses a screw type tensioner applied to chain driven camshaft mechanisms. The tensioner has a shaft rotatably mounted in a cylindrical body and applied with torque by a torsion spring, and an actuating rod having a female thread engaged with a male thread of the shaft. The shaft is urged by the torsion spring in a rotational direction to project the actuating rod. When the tension of a chain decreases as a result of the expansion of the chain, the rod is projected by the rotation of the shaft to push a chain guide, thereby automatically tensing the chain.
When assembling such a spring loaded screw type tensioner, the shaft is rotated to twist the torsion spring to cause a predetermined torsion in the spring and held at a certain position to keep the torsion. On the other hand, the rod is screwed in the innermost portion of the shaft and restrained from rotating relative to the shaft. After attaching the tensioner to a body of the engine and abutting against an arm of a tension pulley, the shaft and the rod are released. Thus, the rod is pressed against the arm by the thrust which is converted from the torsion of the spring.
However, the tensioner is not always assembled to have a predetermined thrust, because of variances in accuracy of the elements such as an engaging portion of the shaft and rod. However, it is difficult to manufacture tensioners without variances in dimension.